DS1 went through a ghastly phase in yr 2, too. Just as I thought things were on an even keel and my very active toddler was maturing into a sensible school boy!! There is a lot of pressure on them to settle into school and be good, in year 1. I wonder if the novelty wears off in year 2 and some of that repressed energy and bounce comes out as naughty/aggressive behaviour?
He told me to F* off which was totally out of character for him. He became aggressive with friends too, and seemed to lose a lot of social skills. He was far worse when tired/overwraught, and although he seemed to have outgrown Christmas excitement a bit, the build up to Christmas at school involves lots of hype, is bound to affect them.
Be firm on your boundaries. Plan ahead on discipline and have appropriate sanctions worked out which will register with him. (DS1, aged 6, totally out of the blue, and out of character, spat at a visitor!!! He still recalls spending the next 4 hours alone in his room.) But don't assume that everything is going wrong for you. This may well just be another phase, and with some clear boundaries and lots of love, he will be back on track sooner or later
(DS1, aged 8, is wonderful again now. Not perfect, but a happy chap, healthily naughty on occasions, but well within expectations)
Best of luck